Voice operated control circuit for two-way signal transmission systems



R. H. HERRICK VOICE OPERATED CONTROL CIRCUIT FOR TWO-WAY April 26, 1949.

SIGNAL TRANSMISSION SYSTEMS 2 Sheets-Sheet 1 Filed April 28, 1944 INVENTOR. ROSWELL H. HERRIOK ATTORNEY April 1949- R. H. HERRICK VOICE OPERATED CONTROL CIRCUIT FOR TWO-WAY S IGNAL TRANSMIS S ION SYSTEMS 2 Sheets-Sheet 2 Filed April 28, 1944 INVENTOR. ROSWELL H HERRICK ATTORNEY I Patented Apr. 26, 1949 VOICE OPERATED CONTROL CIRCUIT FOR TWO-WAY SIGNAL TRANSMISSION SYS- TEMS Roswell H. Herrick, Oak Park, 111., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application April 28, 1944, Serial No. 533,205

8 Claims. (Cl. 179-170) The present invention relates generally to twoway transmission systems which employ separate amplifying means for transmitting signals in each direction and signal controlled switching circuits for controlling the gain of the amplifying means and, more particularly, to improvements in the method of determining the signal direction and controlling the gain of the amplifying means.

In the usual two-way signal transmission system hybrid transformers and balancing networks are used to couple the amplifiers to the transmission line so as to prevent the output of one amplifier from affecting the input of the other amplifier. Such an arrangement will operate very satisfactorily if the impedance of the line remains constant. However, this condition is seldom realized in practice when the line may be exposed to varying weather conditions which materially change the characteristics of the line. When the line characteristics change the hybrid network becomes unbalanced and the input and output circuits of the two amplifiers are no longer completely isolated. In order to prevent singing the gain of the amplifiers must therefore be limited to a value such that the transmission gain is less than the transmission loss, in the loop circuit formed by the two amplifiers and the hybrid transformers, under the worst conditions of unbalance likely to occur.

In order to increase the usable gain of the amplifiers various methods have been proposed for automatically adjusting the gain in response to the arrival of a signal at the input of either of the amplifiers so as to increase the gain of the active amplifier and reduce the gain of the inactive amplifier. A new approach to the problem is disclosed in the present invention in which novel circuit arrangements are employed in place of the usual hybrid network so as to reduce the effect of changes in line characteristics. These novel circuit arrangements permit operation of the amplifiers at a high normal gain. To provide additional stability auxiliary control circuits are employed to maintain both amplifiers partially disabled normally with means for determining the signal direction and. changing the gain of the amplifiers accordingly. In order to avoid any clipping of the first few syllables to be transmitted the control circuit must operate rapidly but should not produce any transient signals as a result of the switching operation. This can be more readily accomplished when the change in gain is limited, as in the present invention, to just that amount required to insure stable operation. It has been found that the thump resulting from transient effects which occur in switching circuits using a variable bias on one or more control electrodes of the amplifier tubes can be avoided by incorporating inverse feedback in the amplifiers and changing the degree of inverse feed back to control the gain.

It is an object of the present invention, therefore, to provide a signal transmission system of the character described which is substantially independent of changes in line characteristics.

It is a further object of the present invention to provide a new and novel method for isolating the input and output circuits of the amplifiers in a two-Way voice repeater.

Another object of the invention is to provide a reliable control circuit that will determine the direction of the signal transmission and will change the gain of the amplifiers accordingly.

A still further object of the invention is to provide a means for changing the gain of the amplifiers without producing objectionable transient effects.

Other objects and features of the invention will appear upon a further perusal of the specification and the accompanying drawings in which;

Fig. 1 discloses an embodiment of the invention employing a relay in the control circuit for changing the gain of either amplifier.

Fig. 2 is a modification of Fig. 1 that employs non-linear circuit elements in place of the relay.

In the illustrated embodiments of the invention separate voice repeating channels are provided for each direction of transmission. The output circuits of each channel are connected to the lines through voltage transforming networks which are illustrated by T pads I0 and I1. Each amplifying channel has a special balancing input stage comprising two identical vacuum tubes with their plate circuits difierentially connected by a transformer coupled circuit to the second stage of amplification. The grid circuits of each balancing input stage are fed with voltages derived from opposite sides of the corresponding voltage transforming network. The voltages which are derived from the voltage transforming network are in phase with each other but the ratio between them depends on the direction of signal transmission through the network. Circuit arrangements are provided so that for one direction of signal transmission through the voltage transforming network the voltages applied to the grids of the vacuum tubes in the con-' nected balancing input stage are equal, but for the opposite direction of signal transmission they are unequal. Due to the differential connection of the plate circuits of the vacuum tubes in the input stage there will be no voltage induced in the second area of the coupling transformer for one direction of signal transmission but there will be a voltage induced in the secondary for signal transmission in the opposite direction. The combination of the voltage transforming network and the balancing input stage therefore accomplishes the same result as the conventional hybrid network but eliminates the use of a balancing network to match the line with its attendant dilficulties which result when the line characteristics change.

The voltages derived from the voltage transforming network are also used to control the auxiliary gain control circuit which increases the gain of the amplifier in the active channel and may also decrease the gain of the amplifier in the inactive channel in accordance with the direction of signal transmission. In the first illustrated embodiment of the invention this is accomplished by means of a three position relay equipped with differentially acting coils. In the second illustration non-linear circuit elements are used in the inverse feedback paths of each amplifier. The alternating current impedance of these non-linear circuit elements is controlled by varying the amount of direct current flowing through them to effect the required change in gains of the amplifiers.

The operation will now be described in greater detail with reference to the accompanying drawings. Referring to Fig. 1, between the lines I and 2 there is provided a channel 3 for amplifying signals transmitted in a direction from line I to line 2, and a channel 4 for amplifying signals transmitted in the opposite direction. The channel 3 comprises the balancing input stage consisting of vacuum tubes 5 and 6, and the amplifying stages consisting of vacuum tubes 1 and 8 with the associated coupling transformers and circuit elements.

Line I is connected to the output transformer 9 of channel It and to the balancing input stage, vacuum tubes 5 and 6, of channel 3 by means of the voltage transforming network Ill. One side of the network It is connected directly to the output transformer 9 of channel 4. The other side of network It is connected to the line coupling transformer II through resistor I2. The line coupling transformer I I is shunted by resistor I3. The purpose of resistor I2 is to make the total effective impedance connected across the line side of voltage transforming network I substantially constant and having a minimum reactive component. To this end the resistor I2 is preferably made large in comparison to the parallel impedance of resistor I3 and the reflected impedance of the line. The value of resistor I3 is chosen so as to offer the proper net matching impedance to the line so as to avoid reflections. This circuit arrangement prevents changes in line characteristics from producing any appreciable change in the phase or magnitude of the voltages which appear across the opposite sides of the voltage transforming network Ill.

The line side of voltage transforming network I0 is connected to the control grid of vacuum tube Ii, the opposite side of the network In is connected to voltage divider I4. The adjustable tap on voltage divider I4 is connected to the control grid of vacuum tube 5. This tap is adjusted so that the voltages applied to the control grids of vacuum tubes and 6 are equal when a 4 signal is transmitted through channel 4 to line I. When a signal is received from line I the signal voltage impressed on the control grid of vacuum tube 6 will be greater than that on the control grid of vacuum tube 5. The plate currents of the vacuum tubes 5 and B vary in accordance with the voltages on the control grids, causing a voltage to be induced in the secondary of transformer I5 which is proportional to the difference in the voltages impressed on the control grids. As these voltages are always in phase the difference will be zero for signals transmitted through network It to line I but will have a value proportional to the magnitude of the signal for signals received from line I. Assuming that a signal is being received from line I, there will be a voltage induced in the secondary of transformer I5 which is amplified by vacuum tubes 7 and 8 and repeated to line 2 through the output transformer it, voltage transforming network II, resistor I 8, and line coupling transformer I9. The resulting voltages produced across the opposite sides of the network i? are applied to the balancing input stage of channel i in the same manner as described for channel 3 in connection with network it. There will not be any voltage induced in the secondary of transformer 45 for signal transmission in this direction and vacuum tubes 2i) and 2! therefore remain inactive. The gain of the amplifier in channel 3 is normally reduced by reason of the inverse feedback path provided from the plate of vacuum tube 8 through condenser 22, resistor 23, contacts 24 of relay 25, and resistor 25 which is included in the control grid circuit of Vacuum tube 3. The voltage produced across resistor 26 as a result of the current flowing through this feedback path opposes the signal voltage impressed on the control grid of vacuum tube 8 by the preceding amplifying tube a due to the 180 degree phase shift in vacuum tube 8, thus reducing the net gain of the amplifier.

The signal voltages developed in the plate circuits of vacuum tubes 5 and 6 are also applied to voltage dividers 21 and 23 through condensers 29 and st, respectively. The taps on these voltage dividers are adjusted so that the signal voltage applied to the control grid of vacuum tube SI is greater than that applied to vacuum tube 32 for equal signal voltages across voltage dividers 2! and 2%. Vacuum tubes 3i and 32 are normally biased to cut-off by battery it. When signal transmission occurs in the direction from line i to line 2 through channel 3 unequal voltages will loe developed in the plate circuits of vacuum tubes 5 and 5 as a result of the sigiiial voltages appearin across the opposite sides of voltage transforming network it. In this case the plate current of vacuum tube 32 will be greater than that of vacuum tube 3i when the voltage dividers El and 23 are properly adjusted thus causing the magneto motive force developed in coil 341' of relay 25 as a result of the plate current drawn by vacuum tube 32 to be greater than that developed in coil 33 of relay 25 as a result of the plate current drawn by vacuum tube iii. The two coils 33 and 34 of relay 25 act differentially on the armature 35 which normally occupies a neutral position. When the current in coil 33 is greater than that in coil as the armature as rotates clockwise so as to operate contacts 36 and 31'. When the current in coil 3 is greater than that in coil 33 the armature 35 re tates counterclockwise so as to operate contacts as and 33. Therefore,'when. the plate current of vacuum tube 32 is greater than that of vacuum tube 3| contacts 24 and 38 of relay 25 will be operated. Contacts 24 open the inverse feedback path in channel 3 and thus increases the net gain of that channel. Contacts 38 connect resistor 42 in parallel with coil 33 which shunts some of the current from coil 33 and thus increases the torque acting on armature 35 to make the relay action more positive. Since the currents that flow in the plate circuits of vacuum tubes 3! and 32 are pulsating in character filter condensers 49 and 41 are provided to smooth out the pulsations so as to prevent the relay 25 from fluttering. When the signal transmission ceases the plate currents of vacuum tubes 3| and 32 return to zero and relay 25 restores to its neutral position. For signal transmission occurring in the reverse direction, from line 2 to line i, the signal voltage developed in the plate circuit of vacuum tube is equal to that developed in the plate circuit of vacuum tube 5. The plate current drawn by vacuum tube 3| will be greater than that drawn by vacuum tube 32 under these conditions, thus causing relay 25 to operate its contacts 36 and 31. Contacts 36 will open the inverse feedback path in channel 4 which is similar to that previously described for channel 3 so as to increase the gain of that channel. Contacts 3? connect resistor 39 in parallel with coil 34 to increase the torque acting on armature 35 as described for resistor 42 and coil 33. t is thus apparent that relay 25 will operate contacts 36 and 3'! for signal transmission through channel 4 and that it will operate contacts 24 and 38 for signal transmission through channel 3 in accordance with the direction of the signal transmission through the voltage transforming network it) so as to open the inverse feedback path in the active channel. The portion of the control circuit which has been described is a complete operative system and it is intended that it may be used in this manner. However, in some instances the operation may be improved by the addition of a duplicate control circuit acting on the same relay but controlled in accordance with the voltages developed across the opposite sides of voltage transforming network l1. This arrangement has been illustrated in Fig. 1 where the duplicate control circuit comprises vacuum tubes 43 and 44' with their associated voltage dividers. This control circuit operates in the same manner as described for vacuum tubes 3! and 32 so that for signal transmission in a direction from line I to line 2 the plate current drawn by vacuum tube 32 is greater than that drawn by vacuum tube 3! and in addition the plate current drawn by vacuum tube 44 is greater than that drawn by vacuum tube 43. The two control circuits thus co-operate to effect the operation of relay 25 in the proper direction. The object of providing a duplicate control circuit under some conditions is to make the circuit symmetrical so that the sensitivity of the control circuit is the same for either direction of signal transmission. Without the duplicate control circuit the sensitivity would be greatest for signal transmission from line 2 to line I due to the amplification which occurs in channel 4 before the signals reach the voltage transforming network ID.

Referring now to Fig. 2, the circuit elements corresponding to those shown in Fig. 1 have been designated with primes. The line coupling circuits and the amplifiers in channel 3' and 4 are identical to those shown in Fig. 1 except for a modification of the inverse feedback path. This path may be traced from the plate of vacuum tube 8' through condenser 22', resistor 23', and resistor 25. In Fig. 1 this path was interrupted by the relay contacts to increase the gain when required, but in Fig. 2 a shunt path is provided around resistance 26. The shunting circuit is through resistor 50, condenser 5|, rectifier 52, and condenser 53 to ground. A similar shunting circuit is provided for channel 4' through rectifier 54 and condenser 53 to ground. Rectifiers 52 and 54 have been illustrated as the dry disc type but any rectifier having a curved characteristic may be employed. It is well known that the impedance of such a rectifier is high for a small applied alternating voltage, but may be made progressively lower bycausing a direct current to flow through the rectifier. A direct current is caused to flow through rectifiers 52 and 54 from battery 55 through resistors 56 and 51, respectively. The control circuit is arranged to increase the direct current flow through rectifier 52 and to decrease the direct current flow through rectifier 54' for signal transmission in a direction from line I to line 2 through channel 3. The alternating current impedance of rectifier 52 is consequently reduced while the impedance of rectifier 54 is increased. The decreased impedance of rectifier 52 shunts a reater portion of current in the inverse feedback path from resistor 26 and thus reduces the degree of inverse feedback in channel 3 so as to increase the gain of that channel. At the same time the degree of inverse feedback in channel 4' will be increased causing a reduction in gain in channel 4'.

The manner in which the direct current in the control rectifiers is caused to vary in accordance with the direction of signal transmission is similar to the control of the relay in Fig. 1. In Fig. 2 separate amplifying tubes preceding vacuum tubes 3| and 32 have been shown although the voltage dividers 27' and 28 could be connected to the plates of vacuum tubes 6 and 5', respectively, in a manner similar to that shown in Fig. 1. Also the separate amplifying tubes 58 and 59 shown in Fig. '2 could be used in a similar manner in Fig. 1. However, when vacuum tubes 58 and 59 are used, as illustrated in Fig. 2, the adjustment of the taps on voltage dividers 21' and 28' is made so that equal voltages are applied to the control grids of vacuum tubes 3| and 32' for equal voltages across the voltage dividers. This adjustment is necessary because in this case the control voltages are derived directly from the voltage transforming network It rather than from the balancing input stage and the effect of voltage divider l4 does not have to be considered in connection with the control circuit. The advantage of using separate amplifying tubes for the control circuit is that the sensitivity of the control circuit is doubled for equal gains in vacuum tubes 5, 5, 58 and 59, but at the expense of additional equipment. For signal transmission in a direction from line I to line 2 the plate current of vacuum tube 3| will be greater than that of vacuum tube 32. The plate current of vacuum tube 3| flows through resistor 60 and the plate current of vacuum tube 32 flows through resistor 6|. The difference in the plate currents flowing in these two resistors causes a difference in potential between points 62 and 63. When the plate current of vacuum tube 3| is the greatest there will be a greater voltage drop in resistor 65 than in 61 and point 63 will therefore be positive with respect to point 62. An additional direct currentcomponent will therefore flow through rectifiers 52 and 54 from point 63 to point 62. This additional current combined with the normal biasing current in the rectifiers causes a net increase in the direct current flowing through rectifier 52 and a net decrease in the direct current flowing through rectifier 54. The impedance of rectifier 52 is consequently reduced causing an increased shunting effect on resistor 26' to reduce the degree of inverse feedback in channel 3. At the same time the impedance of rectifier 54 is increased causing an increase in the degree of inverse feedback in channel 4. For signal transmission in the opposite direction the plate current of vacuum 32' will be greater than that of vacuum tube 3 I and the reverse efiect occurs. Filters 6% and 65 are provided in the plate circuits of vacuum tubes 3| and 32 to eliminate the pulsations in plate current due to rectification of the signal transmission.

The portion of the control circuit thus far described forms a complete operative system and it is intended that it may be so used. Under certain conditions it may be desirable to provide a duplicate set of control tubes acting on the same control rectifiers but controlled from voltage transforming network l1. Therefore, the complete circuit for duplicate controls has been illustrated in Fig. 2. Vacuum tubes 43 and 44' co-operate with vacuum tubes 3'! and 32' to control the direct current flow in rectifiers 52 and 54.

It is to be understood that various modifications may be made in the form of this invention above described without departing from the spirit of the invention as defined in the appended claims.

What is claimed is:

1. In a transmission system, a bi-directional signal current transmission channel including a voltage transforming network operative to pass signal currents in either direction, a second signal current channel including a pair of electron discharge devices each having a control electrode, said electrodes being coupled to said bidirectional channel on either side of said network, means including said network for impressing signal voltages upon said control electrodes so that said electrodes exert substantially unequal efiects on the electron streams of said devices when signal currents traverse said bi-directional channel in one direction and exert substantially equal effects on the electron streams of said devices when signal currents are transmitted over said bi-directional channel in the opposite direction, and means connected to the anodes of said electron discharge devices for differentially combining their outputs whereby signal currents are transmitted from said bi-directional channel through said second channel only when they traverse said bi-directional channel in said one direction.

2. In a transmission system as claimed in claim 1, a second pair of electron discharge devices having control electrodes coupled to the anodes of said first pair of electron discharge devices, means for controlling the voltages impressed upon said second control electrodes so that they exert substantially unequal effects in one sense on the electron streams of said second devices when signal currents traverse said bi-directional channel in one direction and exert substantially unequal efiects in an opposite sense on the electron streams of said second devices when signal currents traverse said bi-directional channel in the opposite direction, and means controlled in accordance with the difference in anode currents of said second pair of electron discharge devices for affecting the transmission efficiency of said second channel.

3. In a transmission system, a bi-directional signal current transmission channel including a voltage transforming network operative to pass signal currents in either direction, a second signal current channel coupled to said bi-directional channel at points on either side of said network, a first means controlled in accordance with the difference between the signal voltages across said bi-directional channel at points on either side of said network for repeating signal currents transmitted thereover in one direction to said second channel, said first means being ineffective to repeat signal currents transmitted through said network in the other direction, and a second means controlled in accordance with the diiference between the signal voltages across said bi-directional channel at points on either side of said network for varying the transmission efficiency of said second channel accordingly.

i. A two-way voice repeating system comprising two lines, two oppositely directed repeater paths connected therebetween for repeating signal currents transmitted over either line to the other line, a voltage transforming network associated with one of said paths operative to iass signal currents in either direction, and means controlled in accordance with the dif ference between the signal voltages across said one path on either side of said network for increasing the transmission eificiency of one or said repeater paths when signal currents are transmitted through said network in one direction and for increasing the transmission efficiency of the other of said repeater paths when signal currents are transmitted through said network in the other direction.

5. A two-way voice repeating system comprising two lines, two oppositely directed repeater paths connected therebetween, a voltage transforming network in one of said paths operative to pass signal currents in either direction, and

means controlled in accordance with the difference between the signal voltages across said one path on either side of said network for simultaneously affecting the transmission efficiency of said repeater paths in opposite senses.

6. A two-way voice repeating system comprising two lines, two oppositely directed repeater paths connected therebetween, a voltage transforming network in one of said paths operative to pass signal currents in either direction, and means controlled in accordance with the difierence between the signal voltages across said one path on either side of said network for simultaneously afiecting the transmission emciency of said repeater paths in opposite senses for one direction of signal transmission through said network and for inversely .afiecting the transmission eiliciency of said repeater paths in opposite senses for the opposite direction of signal transmission through said network.

7. A two-way voice repeating system comprising two lines, two oppositely directed repeater paths connected therebetween, a voltage transforming network in one of said paths operative to pass signal currents in either direction, a nonlinear circuit element in each of said repeater paths effective to control the transmission efficiency of said repeater paths in accordance with the impedances of said circuit elements, and

means controlled in accordance with the difference between the signal voltages across said one path on either side of said network to vary the impedances of said circuit elements in opposite senses.

8. In a transmission system, two lines, two voice repeaters connected therebetzveen for transmitting signal currents from one line to the other in either direction, means associated with each of said repeaters for producing substantially equal signal voltages across predetermined points therein in response to signal currents traversing said system in one direction and substantially unequal signal voltages across said predetermined points therein in response to signal currents traversing said system in the other direction, each of said repeaters being effective to transmit signal currents in accordance with the difference between the signal voltages produced across said predetermined points therein, a control circuit including a pair of electron discharge devices each having a cathode, an anode, and a control REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,212,337 Brewer Aug. 20, 1940 2,282,403 Herrick May 12, 1942 2,379,768 Van Wynen July 3, 1945 FOREIGN PATENTS Number Country Date 496,265 Great Britain Nov. 25, 1938 

